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BUG: KCSAN: data-race in complete_signal / do_group_exit

write to 0xffff888099b2a4f4 of 4 bytes by task 9955 on cpu 0:
 complete_signal+0x50b/0x5c0 kernel/signal.c:1039
 __send_signal+0x675/0x8a0 kernel/signal.c:1175
 send_signal+0x211/0x2a0 kernel/signal.c:1236
 do_send_sig_info+0x59/0xc0 kernel/signal.c:1285
 send_sig_info kernel/signal.c:1604 [inline]
 send_sig+0x59/0x80 kernel/signal.c:1614
 pipe_write+0xc3b/0xd40 fs/pipe.c:500
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0x16a/0x1a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888099b2a4f4 of 4 bytes by task 9951 on cpu 1:
 do_group_exit+0x79/0x1a0 kernel/exit.c:882
 __do_sys_exit_group kernel/exit.c:910 [inline]
 __se_sys_exit_group kernel/exit.c:908 [inline]
 __x64_sys_exit_group+0x2b/0x30 kernel/exit.c:908
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9951 Comm: syz-executor.0 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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